Air flow meter circuit with temperature compensation circuit

ABSTRACT

An air flow meter circuit with a temperature compensation circuit comprises an air flow rate detection circuit for generating a output signal in accordance with an air flow rate, an output circuit for amplifying the output signal of the air flow rate detection circuit to produce an output signal whose value is so adjusted as to be related to the value of the input signal in a predetermined relationship, and a constant voltage circuit for supplying a predetermined reference voltage to the air flow rate detection and output circuits. In order to compensate the output of the air flow meter circuit for its changes due to the influence of temperature coefficients of component parts of both the air flow rate detection and output circuits, the temperature coefficient of the output of the constant voltage circuit is so adjusted as to cancel out the temperature coefficients of the parts constituting the air flow meter circuit.

The present invention generally relates to an air flow meter circuitwith a temperature compensation circuit and more particularly to atemperature compensation circuit for compensating changes in output dieto changes in temperature of the entirety of the air flow meter circuit.

In an internal combustion engine, for example, the intake air flow rateof the internal combustion engine is detected and used as a parameterfor controlling the operation of the internal combustion engine. Fordetection of the intake air flow rate, a hot wire air flow meter, forexample, is available wherein a hot wire heated to a predeterminedtemperature is placed in an intake air path, and a current flowingthrough the heated hot wire is measured to detect an air flow rate. Thistype of air flow meter is disclosed in, for example, U.S. Pat. No.4,297,881 to Sasayama et al issued on Nov. 3, 1981. Since, in an airflow meter, the value of air flow rate to be detected changes with thetemperature of intake air, it has hitherto been practice to compensatethe detected output for intake air temperature. Generally, in a hot wireair flow meter, for example, a cold wire is provided in addition to thehot wire, which cold wire is placed in the same intake air path toeffect the direction of the air temperature simultaneous with thecompensation therefor. Generally, this type of temperature compensationis also employed in a variety of intake air flow meters of the othertypes.

Even with the compensation for the intake air temperature, there stillremains, in practice, a problem that parts constituting the air flowmeter circuit, for example, resistors change in their resistance valuesas the ambient temperature changes. Thus, because of a temperaturecharacteristic of each of the component parts, the relation between airflow rate and output value also has, in practice, a temperaturecharacteristic, i.e., temperature dependence. Especially, in the case ofthe air flow meter for use in internal combustion engines, a flow metermodule is placed in an engine room and hence exposed to large changes intemperature. Therefore, the problem of the temperature dependence isserious.

The inventors have found that such temperature dependence has a greateffect on accuracies of the air flow meter and recognized the necessityof compensation for the temperature characteristic.

In addition, higher accuracy is required for a temperature compensationcircuit of that air flow meter than for general temperature compensationcircuits, because the air flow rate is related to the output value ofthe air flow detection circuit by a fourth-power exponential function aswill be described later with reference to a formula and therefore, inorder to measure a flow rate with 4% accuracy, for example, the accuracyof the detection must be held to be 1%. Furthermore, since thetemperature coefficient of one air flow meter usually differs from thatof another, desirability is such that desired adjustment of thetemperature coefficient can be done with ease and the temperaturecompensation never disturbs the predetermined relation between air flowrate and output value. However, a highly accurate temperaturecompensation circuit has not been materialized heretofore which can meetthe above particular conditions imposed on the temperature compensationcircuit of the air flow meter.

The present invention has been achieved with a view of solving the novelsubject matter found by the inventors and of meeting the necessity oftemperature compensation for the entirety of the air flow meter circuitas well as the necessity of provision of a highly accurate temperaturecompensation circuit.

Accordingly, an object of this invention is to provide an air flow metercircuit subject to the temperature compensation which meets theaforementioned subject matter.

Another object of this invention is to provide a highly accuratetemperature compensation circuit which is simplified in construction andeasy to adjust.

To accomplish the above objects, according to the present invention, anair flow meter circuit incorporates a temperature compensation circuitwhich is simplified in construction and adjustable in order for itstemperature coefficient to be a desired value, whereby the temperaturecoefficients of other circuits than the temperature compensation circuitare compensated so as to zero the temperature coefficient of theentirety of the air flow meter circuit. To obtain the temperaturecompensation circuit of simplified construction according to theinvention, the inventors take advantage of such a characteristic of aZener diode that the temperature coefficient of the Zener voltage (Zenervoltage change/temperature change) varies with Zener current value, torealize a circuit whose temperature coefficient can be set desirably byadjusting the Zener current.

The present invention will be apparent from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram showing the construction of an air flow metercircuit;

FIG. 2 is a circuit diagram showing an air flow meter circuit with atemperature compensation circuit according to an embodiment of theinvention;

FIG. 3 is a circuit diagram useful in explaining the temperaturecompensation circuit of the invention; and

FIG. 4 is a graph showing an actual example of temperature coefficientcharacteristic of a Zener diode.

Referring to FIG. 1, an air flow detection circuit 20 has its outputconnected to the input of a zero-span circuit 30. The detection circuit20 and zero-span circuit 30 are supplied with a predetermined referencevoltage from a regulated voltage source 10.

An air flow to be measured, shown at wavy arrows in FIG. 1, impingesupon an air flow rate detection device, not shown, of the detectioncircuit 20, and a flow rate is detected as an electrical signal. Thedetected signal is inputted to the zero-span circuit 30 where the inputvalue versus the output value is so adjusted as to be in a predeterminedrelationship. Specifically, zero point and inclination of acharacteristic curve representative of the input value versus the outputvalue are determined. Such predetermined relationship of the zero-spancircuit 30 is determined by a request from a separate control circuit,not shown, connected to the output of the zero-span circuit 30.

FIG. 2 shows a preferred embodiment of an air flow meter with atemperature compensation circuit according to the invention. The hotwire air flow meter described previously is used as an air flow meter inthis embodiment. Referring to FIG. 2, power supply voltage V₊ issupplied to the collector of a transistor Tr 1 having its emitterconnected to a hot wire RH placed in an air flow path (not shown). Theother end of this hot wire RH is grounded via a resistor R1. Connectedacross the collector and base of the transistor Tr 1 is a resistor R12.Resistors R2 and R10 are connected, at one end, in common to the emitterof the transistor Tr 1. The other end of the resistor R2 is connected tothe inverting input terminal of an operational amplifier OP1 via aresistor R9. The other end of the resistor 2 is also connected to oneend of a variable resistor R3, the other end of which is connected tothe non-inverting input terminal of an operational amplifier OP4 via aresistor R21. The junction between the variable resistor R3 and resistorR21 is connected with the junction between the hot wire RH and resistorR1. A voltage drop V₂ due to a current flowing through the hot wire RHand resistor R1 is applied to the non-inverting input terminal of theoperational amplifier OP4 via the resistor R21. The other end of theresistor R10 is connected to the inverting input terminal of theoperational amplifier OP1. Both the input terminals of this operationalamplifier OP1 are bridged via a capacitor C5.

The non-inverting input terminal of the operational amplifier OP1 isconnected to a resistor R11 having the other end connected to a resistorR14. A resistor R4 has one end connected to the output of a constantvoltage circuit 100 blocked by chained line and the other end connectedto a resistor R6 and a variable resistor Rr. The other end of theresistor R6 is connected with the inverting input terminal of anoperational amplifier OP2. The non-inverting input terminal of thisoperational amplifier OP2 is connected to the junction between the hotwire RH and resistor R1. Connected across the output terminal andinverting terminal of the operational amplifier OP2 is a seriesconnection of a cold wire RC and a resistor R8. The cold wire RC isplaced at a position where the temperature of air flow to be measuredcan be detected. The output terminal of the operational amplifier OP2 isalso connected to the noninverting input terminal of the operationalamplifier OP1 via the resistor R11 and ground via the resistor R14. Aresistor R7 and a capacitor C1 are connected, at one end, in common tothe inverting input terminal of the operational amplifier, with theother end of each of the resistor R7 and capacitor C1 grounded.

The transistor Tr1, hot wire RH, cold wire RC, resistors R1 to R14,capacitors SC1 and C5, and operational amplifiers OP1 and OP2 constitutea feedback circuit 200 which controls the current I_(H) such that thetemperature of the hot wire RH is kept constant. Thus, the voltage dropV₂ due to the current flowing through the resistor R1 stands for theflow rate detection signal. To be specific, when the air flow impingeson the hot wire, this hot wire is deprived of heat by aerial moleculesand decreased in temperature. An amount of current of the hot wire tocompensate for a decreased temperature thereof corresponds to an airflow rate. This feedback circuit 200 corresponds to the air flowdetection circuit 20 shown in FIG. 1.

Connected to the resistor R4 of this feedback circuit 200 are one end ofa resistor R18 and the output terminal of an operational amplifier OP3,i.e., the output of the constant voltage circuit 100. A variableresistor R19 and a resistor R20 are connected, at one end, to the otherend of the resistor R18. The variable resistor R19 has the other endgrounded and the resistor R20 has the other end connected with theinverting input terminal of the operational amplifier OP4.

The output terminal and the inverting input terminal of the operationalamplifier OP3 are connected together via a resistor R15, with thatinverting input terminal grounded via a series connection of a resistorR15 and a diode D1. The non-inverting input terminal of the operationalamplifier OP3 is supplied with the power supply voltage V₊ via aresistor R27 and grounded via a Zener diode ZD1 in backward connection.A variable resistor R17 is connected between the output terminal of theoperational amplifier OP3 and the cathode of the Zener diode ZD1, andthe junction between the variable resistor R17 and the cathode of theZener diode ZD1 is connected with one end of the capacitor C2, the otherend of which is grounded. The operational amplifier OP3 is also fed withthe power supply voltage V₊ via a resistor R28. The resistor R28 isconnected with one end of a capacitor C3 and the cathode of a Zenerdiode ZD2. The other end of the capacitor C3 and the anode of the Zenerdiode ZD2 are grounded. The capacitor C3 and Zener diode ZD2 are adaptedto protect the operational amplifier OP3 from surge voltage from thepower supply. In this manner, the constant voltage circuit 100 isconstituted.

Meanwhile, the inverting input terminal of the operational amplifier OP4is also connected with a series connection of a variable resistor R22and a resistor R23. The resistor R23 is grounded via a Zener diode ZD3in backward connection. The cathode of the Zener diode ZD3 is connectedvia a resistor R24 to the output terminal of operational amplifier OP4which is grounded via a resistor R25. The resistor R24 is connected withone end of a resistor R26 having the other end connected to an outputterminal V₀. The resistors R18 to R26, Zener diode ZD3 and operationalamplifier OP4 constitute a zero-span circuit 300. Like the operationalamplifier OP3, each of the operational amplifier OP1, OP2 and OP4 isalso fed by the power supply voltage V₊ but for clarity of illustration,the feed line is not depicted.

The operation of the FIG. 2 circuit will be described briefly.

To describe the operation of the feedback circuit 200 in the firstplace, it should be understood that each of the hot wire RH and coldwire RC has a platinum wire wound on an aluminum bobbin and is placed inthe intake air flow path so as to be sufficiently exposed to the flowingair. Both the hot and cold wires have their own resistances exhibiting apositive characteristic with respect to temperatures. In other words,their resistances increase as the temperature rises.

A predetermined amount of current I_(H) is fed from the transistor Tr1into the hot wire RH which in turn is heated to a temperature which ishigher than the temperature of the flowing air by a predeterminedtemperature ΔT_(H). Since the cold wire RC is on the other handconnected to act as a feedback resistor for the operational amplifierOP2 and only an extremely small amount of current is passed through thecold wire RC, the temperature of the cold wire is hardly affected by thecurrent and maintained at the same value as that of a temperature of theflowing air.

The voltage V₂ caused across the resistor R1 by the current flowing fromthe hot wire RH into the resistor R1 is amplified by the operationalamplifier OP2 and fed to the non-inverting input terminal of theoperational amplifier OP1. Since the sum of resistances of the resistorsR2 and R3 is set to be sufficiently greater than a resistance of the hotwire RH, the current flowing in the resistor R1 has substantially thesame value as that of the current I_(H) flowing in the hot wire RH.

The operational amplifier OP2 effects a feedback by an amount which isdetermined by the resistance of the cold wire RC, thereby compensatingthe intake air for its temperature.

The operational amplifier OP1 compares a voltage divided a voltage dropacross the hot wire RH by the resistors R2 and R3 with the outputvoltage of the operational amplifier OP2 to produce a output voltagecommensurate to the difference which in turn is feedback to the hot wireRH via the transistor Tr1, so that the current I_(H) flowing in the hotwire RH is so controlled as to constantly keep the temperature of thehot wire RH higher than the temperature of the intake air by ΔT_(H).

Consequently, as the intake air flow rate changes, the quantity of heatdeprived from the hot wire RH by the intake air changes and then thecurrent I_(H) changes in a sense for cancelling a temperature change ofthe hot wire RH being deprived of heat. Eventually, the current I_(H)changes as a function of the intake air flow rate. Due to the fact thatthe current flowing in the resistor R1 substantially equals 1_(H), thevoltage drop V₂ across the resistor R1 represents the intake air flowrate. Specifically, the amount of intake air flow Q is related to thevoltage V₂ by V₂α Q1/4.

Thus, the voltage V₂ fed to the operational amplifier OP4 is amplifiedthereby to produce a flow rate signal V₀ at the output terminal. Thissignal V₀ may be inputted to a microcomputer for engine control, forexample, and used for air/fuel ratio control.

Next, the operation of the zero-span circuit 300 will be described. Thiscircuit is a non-inverting amplifier using an operational amplifier. Byadjusting the variable resistor R19 connected to the inverting inputterminal, the bias voltage of the operational amplifier OP4 can bevaried to adjust the output signal V₀ to a desired level. Further, byadjusting the variable resistor R22 inserted in the feedback loop of theoperational amplifier OP4, the gain of the non-inverting amplifier canbe varied to desirably set the rate of change of the output signal V₀relative to the input signal V₂, i.e., the input/output characteristic.Thank to the function of adjustments, the output characteristic of theair flow meter can be matched with the specification of a controlcircuit, not shown, fed with the output signal V₀. The Zener diode ZD3provided for the output of the operational amplifier OP4 is adapted toabsorb external high voltage noises which intrude into the zero-spancircuit 300.

The operation of the constant voltage circuit 100 will now be described.This circuit is adapted not only to supply the reference voltage to theoperational amplifiers included in the feedback circuit 200 andzero-span circuit 300 but also to achieve the temperature compensationfor the entirety of the air flow meter circuit which is the subjectmatter of the present invention. In essentiality, the temperaturecoefficient of that reference voltage is adjusted to compensate theentirety of the air flow meter circuit for temperature. For details,reference should be made to FIGS. 3 and 4.

In FIG. 3, the power supply voltage feed line inclusive of the resistorR28 is not illustrated for simplicity. Further, there is noillustration, in FIG. 3, of the parallel connection of the Zener diodeZD2 and capacitor C3 for protecting the operational amplifier OP3 fromsurge voltage, the capacitor C2 for noise protection and the resistorR27 for passage of starting current of the constant voltage circuit uponturn-on of the power supply source, all of which are unessential to thepresent invention.

Taking a Zener diode of HZ 2B-LL type manufactured by Hitachi Ltd., forinstance, a characteristic of temperature coefficient γZ (mV/°C.)relative to Zener current I_(Z) (mA) of the Zener diode playing the partof basis in the principle of the present invention is plotted in FIG. 4,where abscissa represents Zener current in logarithmic scale andordinate temperature coefficient. As will be seen from FIG. 4, thetemperatures coefficient of Zener voltage changes with the Zenercurrent.

From the characteristic curve, the temperature coefficient γZ of theZener diode can be indicated by equation (1):

    γZ=ainI.sub.z +β                                (1)

where α=4.78×10⁻⁴ and β=2.54×10⁻⁴.

The embodiment of FIG. 3 comprises an operational amplifier OP,resistors R_(A), R_(B) and R_(C), a diode D1, and a Zener diode ZD1.Then, denoting a forward voltage of the diode D1 by V_(F), a Zenervoltage of the Zener diode ZD1 by V_(Z), and a set output voltage byV_(S), there results ##EQU1## By neglecting temperature coefficients ofthe resistors R_(A) and R_(B), the temperature coefficient, γS, of theoutput voltage of the constant voltage circuit is given by ##EQU2##where γF represents a temperature coefficient of the diode D1.

It will be seen from equation(3) that the temperature coefficient γS ofthe constant voltage circuit can be adjusted desirably by varying thetemperature coefficient γZ of the Zener diode ZD1. Considering that thetemperature coefficient γF of the diode is generally of the order of -2mV/°C., γZ=-2 mV/°C. may be set by adjusting the Zener current I_(Z)when the temperature coefficient γS of the output voltage V_(S) isdesired to be about 0 mV/°C., for example. The value of γZ=-2 mV/°C. is,however, outside of the controlling range as will be noted from FIG. 4.Then, in order to obtain the value of γS of about 0 mV/°C., the diode D1may be short-circuited (placed out of use) to zero the term of γF inequation (3) and the variable resistor RC may be adjusted so as to setthe Zener current I_(Z) to about 5 mA which makes the γZ substantiallyzero. The circuit of FIGS. 3 or 2 employs the diode D1 because withoutthe diode D1, it is necessary to make the Zener current larger than 5mA, followed by an increase in power consumption in the power supplycircuit, in order to provide the constant voltage circuit with thepositive temperature coefficient. By the use of the diode D1 as in FIG.2, the adjustment for the positive temperature coefficient can be donewith ease for a Zener current I_(Z) which is less than 5 mA. Forexample, to obtain the positive temperature coefficient γS under thecondition that R_(B) /R_(A) =1.0, V_(F) =0.7 V and V_(Z) =2 V, therestands .[.γS=1×(γZ+2) (mV/°C.) (4).]. .Iadd.γ_(S) =1×(γ_(Z) +2)+γ_(Z)(mV/°C) (4) .Iaddend.

From the characteristic curve of FIG. 4, γZ is 0.77 mV/°C. for I_(Z) =1mA and hence .[.γS=1.23 mV/°C. (5).]. .Iadd.γ_(S) =1.23-0.77=0.46(mV/°C) (5) .Iaddend.

is obtained which is positive.

Accordingly, the temperature coefficient γS of the constant voltagecircuit can be desirably set to be positive, zero or negative by varyingthe value of the Zener current I_(Z) through the adjustment of thevariable resistor R_(c).

Since the adjustment of the variable resistor R_(c) has no appreciableeffect on the absolute value of the set voltage V_(S) (actually, theinternal resistance and Zener voltage of the Zener diode are negligiblyslightly increased), the temperature compensation circuit of FIG. 3 canbe incorporated into a circuit requiring a suppressed temperaturedependence characteristic, whereby the variable resistor R_(c) isadjusted for temperature compensation to suppress the temperaturedependence. The FIG. 2 air flow meter circuit of the present inventionimplements the basic principle of the temperature compensation describedthus far.

In the circuit of FIG. 2, the output signal V₀ of the zero-span circuit300 is, ##EQU3## where I_(H) is current flowing in the hot wire RH andV_(Z) is a Zener voltage of the Zener diode ZD1, as describedpreviously, .Iadd.and resistor R₂₃ is assumed to be zero, .Iaddend.

and symbol "||" denotes a parallel resultant resistance of the resistorsR18 and R19 to mean that R18||R19=R18·R19/R18+R19.

Assuming that, ##EQU4## the equation (6) is reduced to .[.V₀ =C·V₂-D·V_(Z) (7).]. .Iadd.V₀ =C·V₂ -D·V_(S) (7) .Iaddend.

where V₂ is a voltage drop across the resistor R1 as describedpreviously.

Pursuant to the King's formula, the relation between the flow ratedetection output V₂ and the air flow rate Q is expressed by

    V.sub.2.sup.2 =A+B∛Q

where Q is in terms of Kg/h, A and B are coefficients, and V₂ is givenby

    V.sub.2 =I.sub.H ×RI                                 (9)

Consequently, the current I_(H) in the hot wire RH is related to the airflow rate Q by a fourth-power root function, and the coefficients A, B,C and D are determined by the resistors constituting the air flow metercircuit.

When the temperature of the air flow meter module changes, theparameters, especially, C, D, I_(H) and R1 change under the influence ofthe temperature coefficients of the component elements constituting thecircuit and as a result, the output signal V₀ of the air flow metercircuit changes. The change in the output signal V₀ can be cancelled byadjusting the temperature coefficient γS of the output voltage V_(S) ofthe constant voltage circuit 100 described with reference to theequations (2) and (3).

Denoting a change in the output signal V₀ due to a change in temperatureby ΔV₀, from the equations (7) and (9) there is obtained .[.ΔV₀ =CR₁ΔI_(H) +I_(H) (CΔR₁ +R₁ ΔA)-ΔD·V_(S) -DΔV_(S) (10).]. .Iadd.ΔV₀ =CR₁ΔI_(H) +I_(H) (CΔR₁ +R₁ ΔC)-ΔD·V_(S) -D·ΔV_(S) (10) .Iaddend.

The output change ΔV₀ of the air flow meter circuit given in equation(10) can be zeroed by adjusting the temperature dependent change ΔV_(S)of the output voltage V_(S) of the constant voltage circuit 100 throughthe adjustment of the temperature coefficient γ_(S).

Practically, the output voltage of the air flow meter circuit placed ina predetermined ambient temperature is first confirmed and thereafter,the air flow meter circuit is placed in a different ambient temperatureand the variable resistor R_(c) is adjusted so that the value of theoutput voltage at condition of the latter ambient temperature equals theoutput voltage at condition of the former ambient temperature.

As has been described, according to the present invention, the air flowmeter free from temperature dependence which is compensated such thatits output signal remains unchanged with variations in the ambienttemperature can be provided, and the highly accurate temperaturecompensation circuit can also be provided. The application of thetemperature compensation circuit is not limited to the air flow meterbut may be extended to various control circuits which similarly disagreewith changes in the output signals due to temperature changes. Inaddition, the temperature coefficient of the temperature compensationcircuit can be desirably set and this characteristic may be applied to acircuit which is required to have a specified temperature coefficient.

Further, the present invention is in no way limited to the embodimentsdescribed thus for but may include many modifications without departingfrom the spirit thereof and scope of claims. For example, the inventionmay be applied to other types of air flow meter than the hot wire airflow meter as exemplified in the foregoing embodiments. Furthermore, theZener current is adjusted by means of the variable resistor connectingthe output of the operational amplifier and the Zener diode in theforegoing embodiments but for adjustment of the Zener current, thevariable resistor may be replaced with a fixed resistor and the fixedresistor may be trimmed. Alternatively, the Zener current may beadjusted by using a separate combination of constant voltage source andcurrent control device.

I claim:
 1. An air flow meter circuit comprising:an air flow ratedetection circuit for generating an output signal in accordance with anair flow rate; an output circuit for receiving the output signal of saidair flow rate detection circuit as an input signal and amplifying theinput signal to produce an output signal whose value is so adjusted asto be related to the value of the input signal in a predeterminedrelationship; a constant voltage circuit for supplying .Iadd.as anoutput signal .Iaddend. a predetermined constant voltage to saiddetection and output circuits; and temperature compensation means foradjusting the temperature coefficient of the output signal of saidconstant voltage circuit such that temperature coefficients of saiddetection and output circuits are cancelled out to substantially zero achange in the output signal .Iadd.of said output circuit .Iaddend. dueto a change in the temperature of the entirety of said air flow metercircuit.
 2. An air flow meter circuit according to claim 1 wherein saidconstant voltage circuit includes a Zener diode whose Zener voltage isused as a reference voltage, and said temperature compensation meansincludes a series connection of a constant voltage source and currentcontrol means, said series connection being connected to said Zenerdiode, said current control means being so adjusted as to vary a Zenercurrent to thereby set the temperature coefficient of the output signalof said constant voltage circuit to a predetermined value.
 3. An airflow meter circuit according to claim 2, wherein said constant voltagesource is provided by the output voltage of said constant voltagecircuit, and said current control means comprises a variable resistor.4. An air flow meter circuit according to claim 3 wherein said air flowrate detection circuit comprises a hot wire air flow rate detectioncircuit including a hot wire heated to a predetermined temperature andundergoing impingement of an air flow to change current flowing in saidhot wire, said current change being detected for measurement of an airflow rate.
 5. An air flow meter circuit according to claim 1 whereinsaid output circuit comprises a non-inverting amplifier circuit havingan operational amplifier, and means for varying feedback rate and inputreference voltage of said non-inverting amplifier circuit such that theinput value versus the output value is so adjusted as to be in thepredetermined relationship.
 6. An air flow meter circuit according toclaim 2 wherein said output circuit comprises a non inverting amplifiercircuit having an operational amplifier, and means for varying feedbackrate and input reference voltage of said non-inverting amplifier circuitsuch that the input value versus the output value is so adjusted as tobe in the predetermined relationship.
 7. An air flow meter circuitaccording to claim 3 wherein said output circuit comprises anon-inverting amplifier circuit having an operational amplifier, andmeans for varying feedback rate and input reference voltage of saidnon-inverting amplifier circuit such that the input value versus theoutput value is so adjusted as to be in the predetermined relationship.8. An air flow meter circuit according to claim 4 wherein said outputcircuit comprises a non-inverting amplifier circuit having anoperational amplifier, and means for varying feedback rate and inputreference voltage of said non-inverting amplifier circuit such that theinput value versus the output value is so adjusted as to be in thepredetermined relationship.
 9. An air flow meter circuit according toclaim 5 wherein the output voltage of said constant voltage circuit isused as the reference voltage of said operational amplifier.
 10. An airflow meter circuit according to claim 6 wherein the output voltage ofsaid constant voltage circuit is used as the reference voltage of saidoperational amplifier.
 11. An air flow meter circuit according to claim7 wherein the output voltage of said constant voltage circuit is used asthe reference voltage of said operational amplifier.
 12. An air flowmeter circuit according to claim 8 wherein the output voltage of saidconstant voltage circuit is used as the reference voltage of saidoperational amplifier.